Method of seamless roaming between wireless local links and cellular carrier networks

ABSTRACT

A scheme that enables seamless roaming between the WLAN and the cellular carrier network by enabling a user that originates a call in the WLAN and happens to go outside the range of the WLAN to automatically switch over to the cellular carrier network without losing connection with the other party. This solution assumes that the mobile device has the capability to operate in at least two modes that include the WLAN mode and one of the cellular carrier modes, such as the GSM, IS-95 CDMA, IS-136 TDMA, and iDEN.

RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 12/874,183, filed on Sep. 1, 2010, now pending,which is a divisional application of U.S. patent application Ser. No.10/688,608, filed on Oct. 17, 2003, now U.S. Pat. No. 7,835,751, whichclaims the benefit of provisional application Ser. No. 60/419,674 filedon Oct. 18, 2002, all of which are incorporated by reference herein inits entirety.

BACKGROUND OF THE INVENTION

The proliferation of the wireless local area networks (WLANs) has led tothe search for ways in which its utilization can be increased. By WLAN,we include all instantiations of such technologies as 802.11 a, 802.11b, 802.11 g, Bluetooth and any similar WLAN versions. For example, thespecification for the IEEE 802.11 version of WLAN includes an accessscheme called the distributed control function, which permits thenetwork to support both data and voice applications. Today voice overwireless local area network (VoWLAN) is a reality. The voice may beencoded and transmitted using voice over internet protocol (VoIP) formatand protocols such as G.711, G.726, G.729, SIP, MEGACO, H.323, or othersimilar protocols that are being developed.

One of the issues in WLAN is that there is a limited range of operationdue to power requirements. For example, a typical IEEE 802.11 WLAN has arange of at most 300 yards from the access point that connects themobile devices to the wired LAN. Thus, when an IEEE 802.11-based mobiledevice roams beyond this range, any call in progress is forciblyterminated.

SUMMARY OF THE INVENTION

The present invention concerns a scheme that enables seamless roamingbetween the WLAN and the cellular carrier network. In an embodiment, thecellular carrier network is a wide area wireless network. The methodenables a user that originates a call in the WLAN and happens to gooutside the range of the WLAN to automatically switch over to thecellular carrier network without losing connection with the other party.This solution assumes that the mobile device has the capability tooperate in at least two modes that include the WLAN mode and one of thecellular carrier modes, such as the GSM, IS-95 CDMA, IS-136 TDMA, andiDEN.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis has instead been placed upon illustrating theprinciples of the invention. Of the drawings:

FIG. 1 is a block diagram showing the architecture of a typical WLANthat supports Voice communications;

FIG. 2 is a timing diagram showing the initial mobile deviceregistration process;

FIG. 3 is a timing diagram showing the handoff procedure for a mobiledevice with a PSTN-based call in progress;

FIG. 4 is a timing diagram showing the handoff procedure for a mobiledevice with cellular network-based call in progress; and

FIG. 5 is a timing diagram showing the procedure for cellularnetwork-to-WLAN handoff.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A typical WLAN is comprised of access points (APs) that are connected tothe enterprise LAN via an access controller (AC) or a wireless LANswitch. In an embodiment, the WLAN may include wireless local links tovarious devices. The access controller/wireless LAN switch AC is thecenter of intelligence of the WLAN and is responsible for admissioncontrol, authentication and mobile device roaming coordination. In anembodiment, the access controller/wireless LAN switch AC may include acomputing device. One AC/wireless LAN switch can control several APs andmultiple ACs/wireless LAN switches can be in the same network. Anotherdevice called the Cellular Proxy (CP) is a gateway that connects theenterprise LAN to the cellular network. In an embodiment, the CP mayinclude a computing device. If the enterprise PBX is not VoIP-capable,the cellular proxy also provides the VoIP interface between theenterprise LAN and the PBX. The Cellular Proxy is located in the part ofthe enterprise building that has a very good cellular coverage.

The Cellular Proxy is not necessarily tightly coupled to any cellularnetwork. To any cellular network, it is a bank of radios. It hides thedetails of the movement of the mobile devices within the enterprisepremises from the cellular network thereby preventing the cellularnetwork from making frequent updates to its database. Also, it canconnect to multiple cellular networks simultaneously because it containsradios for different types of cellular network technologies, such asCDMA, AMPS TDMA, GSM TDMA, iDEN, WCDMA, CDMA2000, GPRS, 1XRTT, 1xEVDO,and 1xEVDV. Thus, the Cellular Proxy can proxy for mobile devices inCDMA-based and TDMA-based cellular networks simultaneously.

Mobile devices, such as laptops that are equipped with wireless networkinterface cards and personal digital assistants access the network viathe APs. A typical network is shown in FIG. 1. In the figure, the accesscontroller and/or wireless LAN switch are not shown; they are assumed tobe part of the enterprise LAN infrastructure.

Each dual-mode mobile device MD has two telephone numbers: one number isassigned by the enterprise as an extension of the enterprise's PBX, andthe other number is assigned by the cellular carrier. The defaultnetwork for each mobile device is the enterprise (or hotspot) WLAN,which means that when a mobile device is turned on, it first searchesfor the enterprise's WLAN and registers with the network, if it isfound. If the enterprise WLAN is not found the mobile device MD thenregisters with the cellular carrier network. The rationale for thisoperational requirement is to save the enterprise money by ensuring thatmobile device users do not accrue air charges when they are within thecorporate WLAN. All calls generated by the mobile device while it is onthe enterprise network are routed via the corporate PBX to the publicswitched telephone network (PSTN).

Since each mobile device has two telephone numbers, it can be reached intwo ways: via the PBX when calls come from the PSTN, and via theCellular Proxy CP when calls come from the Cellular Proxy. Regardless ofhow the calls arrive, they are converted into voice over IP (VoIP)packets by the appropriate device and presented to the mobile device viathe WLAN. These calls are based on the Session Initiation Protocol(SIP), which has the advantage over the ITU-T H.323 protocol in that itis a lightweight protocol that leverages the Internet protocols.

Each mobile device MD goes through an initialization process when it isturned on in the WLAN. As discussed earlier, the default network is theenterprise WLAN. Therefore, when a mobile device is turned on it sends aRegistration Request message to the appropriate authentication server inthe enterprise LAN. After the authority server has authenticated themobile device, it returns a Registration Complete message to the device.The authentication server, which has information on each mobile device'scellular service provider's network, then sends a Registration Requestmessage to the Cellular Proxy instructing the latter to register themobile device in the device's cellular carrier network. The CellularProxy CP first sets up a TCP connection to the mobile device MD via theappropriate access controller (or wireless LAN switch) and access pointbefore commencing the registration of the device in the cellularnetwork. After the Cellular Proxy has successfully registered the mobiledevice in the cellular network, it returns a Registration Completemessage to the authentication server. The Cellular Proxy then startslistening on the appropriate paging channel for calls destined for themobile device from the cellular carrier network and will deliver suchcalls to the mobile device via the appropriate access controller (or thewireless LAN switch) and access point. The message flow for theregistration process is illustrated in FIG. 2.

Consider a dual-mode mobile device that originates a call within a WLAN.As the user moves closer to the edge of the network the signal qualitybegins to degrade. The degradation will reach a point where the signalstrength is almost imperceptible, which causes the call to beterminated.

Here, the mobile device MD has the capability to monitor the signalquality by measuring the signal-to-noise ratio (SNR). Assume also thatfrom practical experience acquired through measurements it is known thatwhen SNR reaches some threshold value d, the voice quality becomesunacceptable. The goal is to prevent the call quality from degrading tothis critical point. Thus, when the SNR drops to a cutoff value r>d, thesystem initiates a handoff with the objective of completing the handoffprocedure before the SNR drops down to the threshold value d. Thus thescheme operates in the following manner:

When a mobile device experiences SNR measurement value of r, it sends aHandoff Request message to the Cellular Proxy via the TCP connectionthat exists between the two devices.

When Cellular Proxy receives the message it takes one of two actionsthat depend on where the device of the other party in the call islocated.

If the device of the other party is located in the PSTN, which meansthat the call passes through the PBX, then it takes the followingactions: 1) the Cellular Proxy uses one of its own carrier assignedtelephone numbers to call the mobile device's carrier-assigned telephonenumber; 2) since the mobile device is not physically connected to thecellular network, the Cellular Proxy will also receive the call onbehalf of the device; 3) after receiving the parameters of the call fromthe cellular network, such as the channel or code to use, power level,etc., the Cellular Proxy will forward these parameters to the mobiledevice over the TCP connection that it established between the two andcommands the mobile device to switch its radio to the cellular networkusing those parameters; 4) the Cellular Proxy will then close the TCPconnection, stop proxying for that device in the cellular network toavoid cloning problems, and will thereafter forward the call to themobile device over the new connection established via the cellularnetwork; and 5) on receiving the call parameters, the mobile device willimmediately switch its radio to the cellular network without having toregister again since it has already been registered and authenticated inthe cellular controller by the Cellular Proxy. As it moves from basestation to base station outside the enterprise network, the mobiledevice will be subject to the handoff mechanism that applies within thecellular network.

This sequence of activities is illustrated in FIG. 3.

If the device of the other party is located in the cellular carriernetwork, which means that the call passes through the Cellular Proxywithout reaching the PBX, it takes the following set of actions: 1)since the Cellular Proxy has all the parameters of the call, it returnsthese parameters to the mobile device via the TCP connection between thetwo and commands the mobile device to switch its radio to the cellularnetwork using those parameters; 2) the Cellular Proxy will then closethe TCP connection and stop proxying for the mobile device to avoidcreating cloning problems in the cellular network; 3) Upon receiving theinformation, the mobile device will tune its radio to the channelcurrently used by the Cellular Proxy and receive the call directly fromthe cellular network.

This sequence of activities is illustrated in FIG. 4.

Assume that the mobile device has a call in progress as it is moved fromthe outside world into the enterprise premises. The operational rule isthat if there is good cellular coverage within the enterprise premises,the call will be allowed to complete in the cellular network after whichthe mobile device will register in the enterprise LAN using theprocedure described earlier. However, if there is no good coveragewithin the enterprise premises, the device will initiate a handoff withthe Cellular Proxy. The procedure is as follows:

1) if the SNR reaches the predefined cutoff value, the mobile devicesends a short message service (SMS) message to the Cellular Proxy. Themessage contains information on the identity of the mobile device, suchas its PBX extension and its cellular network telephone number as wellas the parameters of the current call. The Cellular Proxy maintains arecord of the cellular network that each enterprise mobile device isassociated with. Therefore, with the information it received from themobile device the Cellular Proxy sends a message to the authenticationserver to expedite the authentication of the mobile device.

2) The authentication server will provide emergency registration for themobile device by broadcasting a Registration Invite message that themobile device will respond to.

3) After locating and authenticating the mobile device, theauthentication server forwards the device's location and networkconfiguration parameters like the IP address to the Cellular Proxy.

4) After sending the emergency registration request to theauthentication server, the Cellular Proxy will start monitoring thechannel on which the mobile device was communicating and accumulatinginformation destined for the device until the device has beenauthenticated and registered in the network

5) When the Cellular Proxy receives information on the device'slocation, it will set up a TCP connection to the device and forward allaccumulated packets to the device.

6) After this, the operation becomes similar to that described earlier.The Cellular Proxy listens on the channel and relays information betweenthe mobile device and the cellular network until the conversation isover and the connection is terminated. When the current call ends, theCellular Proxy continues to listen on the cellular network's pagingchannel for calls destined for the mobile device, as described earlier.

This sequence of activities is illustrated in FIG. 5.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A method comprising: facilitating, at a computingdevice, a communication session between a first device and a seconddevice, wherein the computing device is local to the first device duringat least a portion of the communication session; and switching, at thecomputing device, the communication session from a first communicationpath to a second communication path, wherein the first communicationpath passes through a wireless local link, wherein the secondcommunication path passes through a wide area wireless network, whereinthe communication session comprises voice data transmitted via sessioninitiation protocol (SIP), and wherein the voice data comprises voiceover IP (VoIP) packets, and wherein the switching is controlled by thecomputing device.
 2. The method of claim 1, wherein the computing devicecomprises a cellular radio interface configured to interface thewireless local link.
 3. The method of claim 1, wherein the wirelesslocal link is part of a wireless local area network.
 4. The method ofclaim 3, wherein the computing device comprises a cellular radiointerface configured to interface the wireless local link.
 5. The methodof claim 3, wherein the computing device comprises a cellular radiointerface configured to interface the wireless wide area network.
 6. Themethod of claim 1, further comprising in response to a characteristic ofthe communication session across the first communication path exceedinga predetermined threshold, initializing the switching of thecommunication session.
 7. The method of claim 6, wherein thecharacteristic comprises a signal-to-noise ratio of the communicationsession across the first communication path.
 8. The method of claim 6,wherein initializing the switching comprises receiving a handoff requestmessage at the computing device.
 9. The method of claim 1, whereinswitching the communication session from a first communication path to asecond communication path comprises sending parameters of thecommunication session to the first device.
 10. The method of claim 1,wherein switching the communication session from a first communicationpath to a second communication path further comprises instructing thefirst device to switch its radio to the wide area wireless network basedon the parameters.
 11. The method of claim 1, wherein the computingdevice is located at a building, and wherein the first device is locatedat the building during at least a portion of the communication session.12. A gateway device comprising: a radio terminal; and a computingdevice connected to the radio terminal and configured to: facilitate acommunication session between a first device and a second device,wherein the first device is initially local to the gateway; and switchthe communication session from a first communication path to a secondcommunication path, wherein the first communication path passes througha wireless local link, wherein the second communication path passesthrough a wide area wireless network, wherein the communication sessioncomprises voice data transmitted via session initiation protocol (SIP),wherein the voice data comprises voice over IP (VoIP) packets, andwherein the switching is controlled by the gateway.
 13. The gateway ofclaim 12, wherein the gateway comprises a cellular radio interfaceconfigured to interface the wireless local link.
 14. The gateway ofclaim 12, wherein the wireless local link is part of a wireless localarea network.
 15. The gateway of claim 14, wherein the gateway comprisesa cellular radio interface configured to interface the wireless locallink.
 16. The gateway of claim 14, wherein the gateway comprises acellular radio interface configured to interface the wireless wide areanetwork.
 17. The gateway of claim 14, wherein the gateway device is partof the wireless local area network.
 18. The gateway of claim 12, whereinthe gateway is further configured to initialize the switching of thecommunication session in response to a characteristic of thecommunication session across the first communication path exceeding apredetermined threshold.
 19. The gateway of claim 18, wherein thecharacteristic comprises a signal-to-noise ratio of the communicationsession across the first communication path.
 20. The gateway of claim18, wherein initializing the switching comprises receiving a handoffrequest message from the first device to the gateway.
 21. The gateway ofclaim 12, wherein switching the communication session from a firstcommunication path to a second communication path comprises sendingparameters of the communication session to the first device.
 22. Thegateway of claim 12, wherein switching the communication session from afirst communication path to a second communication path furthercomprises instructing the first device to switch its radio to the widearea wireless network based on the parameters.
 23. The gateway of claim12, wherein the gateway is further configured to locally connect withthe wireless local link.